1. Field
Embodiments of the present invention relate to sensors and, in particular, to pressure and shear sensors.
2. Discussion of Related Art
Investigation in tactile sensing has been an active research area for the past 30 years. The technologies applied to date in the tactile sensing field include metal strain gages, conductive elastomers, carbon fibers, conductivity measurement (usually in association with elastomers), ferroelectric polymers (i.e., PVF2), semiconductor strain gages, magnetostriction, capacitance and optoelectronics. The characteristics of all tactile sensors depend, to some degree, on the properties of the deformable contact material. Each material has its advantages and disadvantages, depending on physical properties and manufacturing concerns. Conventional metal strain gages, for example, measure strain based on the induced resistance change. Strain may be defined as the amount of change in length divided by the original length (Δl/l) The advantage of metal strain gages is that they are inexpensive, commonly used in industry, and they have a wide range of sensitivity. The disadvantages of metal strain gages in the use of measuring skin tractions are that they are not suitable for large deformation applications and that the gage factors are too small (two to four for platinum and −12 to −20 for nickel) yielding a low sensitivity. In addition, these gages are not suitable for use in arrays since they would require a large amount of supporting circuits and specific interconnecting wires.
Semiconductor strain gages, however, can measure very small strains (˜1 μm/m) and have a very high gage factor (>150 for Ge and n-type Si). The most widely used semiconductor gages are piezoresistive based silicon devices. These sensors have a very linear mechanical and electrical response almost free of any noticeable hysteresis effect. They also have a relatively low thermal expansion coefficient (Si=3.5×10−6/° C.) compared to metals. The disadvantages include its breaking stress (ranging from 0.41-2.1 giga Pascals (GPa), depending on the diameter of the deformable element). Stress may be defined as total force divided by area (f/A). The devices are relatively stiff (Young's modulus around 130 GPa), and can handle a maximum strain around 0.5%. These devices are therefore mainly for micro strain measurements, and are not intended for large deflections. Thus, the use of an array of silicon sensors for measuring skin tractions is not feasible.
Piezoelectric materials are also commonly used for strain/stress measurement. A piezoelectric material generates an electrical charge when subjected to mechanical stress. The most widely used piezoelectric materials are electric polymers, such as polyvinylidene difluoride (PVDF or PVF2). The advantage of using PVF2 film is that it is flexible and can withstand rather large strains without severe deterioration. Films can be manufactured in thickness ranging from a few microns to a few millimeters. However, the material is structurally weak and prone to damage. In addition, the material suffers from poor fatigue life and from shrinkage due to aging and temperature.
Conductive elastomers are another type of polymer that can be made electrically active either by the addition of metallic compounds or by formation in the presence of high electric fields. These materials offer high resiliency and resistance to corrosion. However, they are highly nonlinear in their electrical and mechanical response and are often mechanically and thermally unstable. Examples include carbon-filled liquid silicone rubber and the conductive polymer in the commercially available F-SCAN force sensitive resistor (TekScan, Inc., Boston, Mass.). The principle of the F-SCAN sensor is based on the fact that the resistance between two intersection points of two conductive polyester sheets is sensitive to contact force. This type of sensor can only be used in special circumstances because of its nonlinear response, hysteresis, and gradual voltage drift.
Another means of transducing force is the use of optical fiber. Optical sensors are unaffected by electromagnetic field interference and can be made relatively compact with a diode source and detector. Optical sensors are also known for their sensitivity and high dynamic range. Furthermore, the sensors can be embedded in most structures with minimal modification. The optical sensors do not suffer from hysteresis and drift, and their response tends to be highly linear. However, there are no available optical sensors that can be used to measure distribution of pressure and shear over a surface. Current optical sensors all use a single optical fiber and are intended for single point measurement of strain or pressure.
There are currently no flexible high-resolution sensors capable of measuring the distribution of both shear and pressure at the plantar interface. As mentioned earlier, one method to measure shear at point locations is to use magneto-resistive transducer disks (16 mm in diameter and 3 mm thick) mounted in an insole that is directly placed under three critical stress regions under a foot (e.g. heel, first and third metatarsals). The sensor's resistance varies with the strength of magnetic field in which it is placed. Lateral movement corresponding to shear force can be monitored by the movement of a magnet that is placed centrally above a center tapped magneto resistor in a bridge configuration.
A piezoelectric film-based sensor using copolymer PVdf-TrFE has also been studied. Again the sensors are few in number and are placed only in critical locations. No shear distribution over the plantar interface can be measured using this sensor. More recently there are developments in distributive shear and pressure sensors using an integrated capacitive sensor and a strain gage sensor.
For the capacitive sensor, one of the problems is its susceptibility to electrical interference because of its high impedance. Strain gages, on the other hand, require additional structure to extract the shear component. Both designs suffer from low spatial resolution, drift, and a high sensitivity to temperature. Another severe limitation is that compliance of these sensors is not commensurate with skin when configured to measure shear. This makes them unsuitable for use as in-shoe shear sensors as they will affect the stresses they are intended to measure. In order to address these challenges, we propose to develop a novel means of transducing plantar pressure and shear stress using a distributive Bragg grating based polymeric waveguide sensor array.